A balanced crystal oscillator circuit can be employed in various electronic circuits. However, a special field of application is within communications equipment—especially telecommunications equipment—wherein frequency references are generated or synthesized as periodic signals for modulators, demodulators, up- and down frequency converters, for timing circuits etc. Typically, the periodic signals are square-wave signals.
The periodic signals have a fundamental frequency and are generated by a so-called frequency synthesizer, which provides the fundamental frequency of the periodic signals as a multiplicity of the fundamental frequency for a reference signal. In order to meet requirements, related to frequency stability or timing precision, set forth in a standard, which the equipment is intended to comply with, a crystal oscillator is typically required for providing a sufficiently stable or precise reference signal.
Communications equipment involves signal processing within different frequency ranges; the circuits arranged to process signals related to modulation of an RF carrier signal operate within the highest frequency ranges and are typically denoted RF stages. Circuits arranged to process signals related to the signal to be communicated on the carrier signal operate within the lower frequency ranges and are denoted base band stages or base band circuitry.
The RF stages are primarily involved with processing signals at relatively high power and at relatively high frequencies; hence heavy noise sources exist as an unavoidable inheritance from the RF stages. The base band circuitry typically involves lower frequency signals at lower power levels, however the signal processing in this circuitry is typically carried out as digital signal processing and hence involves heavy digital switching. The base band circuitry is thus also a vigorous noise source.
For communications equipment providing e.g. wireless communication at Radio Frequencies (RF) a so-called RF frequency synthesizer is provided with the reference signal from a crystal oscillator running typically at 10-40 MHz.
For relatively complex and compact communications equipment such as cellular telephones, Bluetooth™ communication devices etc., ever tighter integration levels makes it desirable to integrate the various circuits of a communications device on a single integrated circuit. Such an integrated circuit is typically of the semiconductor type, where a semiconductor chip (a silicon substrate) is accommodated on a ceramic substrate (or a so-called metal header) in a package with terminals for obtaining electrical contact with a Printed Circuit Board (PCB). Electrical contact between the silicon substrate and the ceramic substrate is accomplished by means of bonding wires. Similarly, electrical contact between the ceramic substrate and the terminals of the package is also accomplished by bonding.
As a consequence of the above desired integration of the circuits and the individual properties of the circuits with respect to generation and emission of noise, the oscillator circuit is situated in a very noisy environment. Hence, the oscillator is prone to pick up interference from the silicon substrate of the integrated circuit. Additionally, since resonator components of the oscillator circuit often are placed outside the integrated circuit (i.e. outside the integrated circuit package), the oscillator is also prone to pick up interference by means of the bonding wires and the terminals. Indeed the crystal is very stable and is to some extent able to suppress the interference, but since requirements are very tight, reduction of interference sensitivity is an ongoing challenge for circuit designers. For a GSM/GPRS cellular terminal, as little as 0.1 ppm frequency shift as a maximum is acceptable, for example in a situation where a typical component such as a Voltage Controlled Oscillator VCO switches on and off or changes frequency.
However, the major cause of undesired frequency shifts or frequency error is due to a shift in the operating point (because of interference-induced DC-voltage drops in common supply wires or due to rectification of interference signals causing local DC shifts in current or voltage and/or due to interference signals entering the crystal oscillator circuit) where non-linear components are modulated (related to changes in the device gm or input capacities). To some extent, on-chip shielding may be used, but the bonding wires and metal tracks are exposed to magnetic fields. A balanced structure as opposed to a single-ended structure helps, but even common-mode (i.e. exposures affecting both of a pair of balanced signals) interference components are also harmful.
Generally, it should be noted that a crystal oscillator provides a very stable oscillator frequency despite variations in supply voltage level and in load characteristics and is thus a very robust oscillator type.